It has become increasingly widespread to utilize electronic systems for detecting the impact of a bullet against a target for rifle and pistol competitions or tournaments and rifle and pistol ranges, e.g. in training or preparation for competitive events.
While many different techniques have been proposed for this purpose, the most common utilize groups of sensors, e.g. mechanical/electrical transducers, spaced around a target region and positioned and electronically connected to respond to the acoustic or shock wave on impact at times which can vary in accordance with the relative positions of the impact and the transducer. The electronic circuitry to which these transducers are connected can then evaluate the distance of the impact from the reference point along a coordinate system and signal, for example, the spacing between the hit and the center of the target.
In, for example, Swiss patent No. 526,763, a pair of sensors are arranged on the periphery of a circle concentric with the center of the target, the sensors of the pair being diametrically opposite one another across the center.
The sensors are thus in clearly defined positions relative to a polar coordinate system whose zero point or origin lies at the target center.
If the sound propagation velocity is known, the impact site can be calculated by a computer, e.g. a microprocessor, forming part of the electronic circuitry, from the time-staggered arrival of the shock waves at the different sensors.
A Swiss patent No. 589,835 proposes arranging acoustic sensors in the target plane utilizing them in a similar manner to calculate the impact position based upon the sound propagation velocity in the target.
Customarily the target comprises an image surface carrying the target pattern, e.g. on a fabric layer, behind which a space is formed. Since the transducers are arranged in this space or compartment, it is sound propagation velocity in this region which determines the response of each sensor to the shock or acoustic wave.
Of course, under fixed conditions, the sound velocity can be readily determined.
In practice, however, it is found that a predetermination of the sound velocity is neither possible nor practical.
The sound propagation velocity is dependent on the temperature of the air and, more particularly, is proportional to the square root of the absolute temperature T in Kelvin, the proportionality constant being ##EQU1## so that ##EQU2## where C is the propagation velocity in m/sec. T is=.crclbar.+273.14 where .crclbar. is the ambient temperature in .degree.C.
It has been found, in practice, that the temperature gradient in the space behind the target, in conventional systems, is nonlinear and nonuniform. It may constantly change as a function of solar radiation angle and solar radiation intensity, wind velocity and direction, the nature of the paint on the target and dark and light zones on the target image, and, of course, changes in ambient temperature conditions.
It has also been found in practice that all of these variables cannot be accounted for each time the target is to be used and from one use to the next. Furthermore, these variables can cause inaccuracy in results which exceed the tolerance limits established by the international associations conducting shooting competitions such as the U.I.T. (Union International de Tir).